A solid state imager can be viewed as including a number of laterally offset pixels containing photosensitive regions. Arrays of color filter elements transmitting selected portions of the visible spectrum and placed in registration with the pixels of a solid state imager are well known to the art and can take a variety of forms. Each color filter element transmits a portion of the color spectrum of incident light into the associated pixel of the imager in order to provide the imager with means of color sensitization. All visible image sensors, including linear sensors, can utilize color filters whether or not the photosensitive region occupies the entire pixel area. Typically, the color filter elements are transmissive of a set of primary colors such as red, green and blue or of complementary colors such as cyan, yellow, and magenta and or white. Lens arrays integral to the image sensor, commonly made by thermal deformation of photolithographically defined polymers, are often employed over color filter arrays to direct light rays through color filter elements to the photosensitive regions.
FIG. 1 shows a single pixel 10 of a typical interline image sensor in registration with color filter elements 24a and 24b partially including a color filter array, lens 22, photodiode 14 formed in semiconductor substrate 12, gate electrode 16, and light shield 18. The gate electrode and light shield are typically isolated electrically from one another and from the substrate by isolation oxides not shown. A color filter array includes a plurality of color filter elements 24a, 24b, and 24c (not shown), typically provided in a pattern of three or more elements each transmitting a different spectral region. Photodiode 14, semiconductor substrate 12, gate electrode 16, and light shield 18 form semiconductor portion 40 of the imager.
The prior art image sensor with a pixel shown in FIG. 1 has a partially planarizing layer 20a for offsetting the color filter element 24a from photodiodes 14. Conventionally, the partially planarizing layer includes an organic spacer layer spin coated to achieve some degree of planarization in order to provide simpler processing conditions for deposition of the color filter array, such as the color filter arrays described in Nomura, U.S. Pat. No. 5,321,249, typically coated from organic materials. Partially planarizing layer 20a enables better process control of the thickness of the color filter elements, essential in controlling the spectral transmission characteristics. The use and limitations of such planarizing layers in optically active solid state image sensors is taught by McColgin, U.S. Pat. No. 4,553,153 for a polymerizable monomer. Upper planarizing layer 20b in FIG. 1 is typically used to space lens 22 away from photodiode 14 and partially compensates for irregular topography of conventional color filter elements.
As shown in FIG. 1, prior art color filter arrays suffer to some degree from lack of planarity, due to both lack of planarity of the substrate on which they are formed (region 60 of FIG. 1 and of FIGS. 2A and 2B) and lack of planarity of the color filter elements relative to one another (region 62 of FIG. 1 and of FIGS. 2A and 2B). Lack of planarity of the substrate produces variations in the thickness of the color filter elements coated on the substrates, which in turn causes local differences in the optical transmission characteristics within each element. Lack of planarity between color filter elements (region 62 of FIG. 1 and of FIGS. 2A and 2B) also results in inter-pixel nonuniformities and in addition causes unwanted light piping and color mixing in regions where a subsequently defined color filter element overlaps a previously defined color filter element. Hartman, U.S. Pat. No. 4,315,978, teaches a method of making color filter arrays in which color filter elements are formed by creating dyeable islands separated by dye impermeable polymers. Neighboring color filter elements, however, overlap to some degree, altering spectral transmission characteristics of color filter elements, and the process typically relies on mask to mask alignment accuracy, thereby reducing process latitude. Spectral characteristics are similarly difficult to control, particularly if the pixel size is small, because the area of misalignment must be reduced with pixel size to maintain adequate color resolution. In addition, the overlapping regions make subsequent coatings difficult to coat smoothly.
Various approaches have been undertaken to improve the planarity of the color filter arrays and the substrates on which they are formed, but none has been fully satisfactory. Nomura, U.S. Pat. No. 5,321,249, relies on spin-on color filter materials which are to some extent self-planarizing. When the materials for the second or the third color filter elements are coated, this technique greatly reduces the thickness of the spun-on material left over the previously deposited color filter elements (region 62 of FIG. 2B) because the previously deposited elements are topographically high, as is well known in the art of spin-on planarization. However, not all the material is removed. Horak, U.S. Pat. No. 4,204,866, teaches a method of making color filter elements in which a single mordant layer is dyed through openings in the photoresist, the process being repeated using dyes of different colors to provide side-by-side color filter elements. However, the color filter elements are not self aligned, so that the process is sensitive to mask to mask misalignment tolerances. Also, the mordant swells when dyed, as is well known in the art, so that the surface of the single mordant layer becomes non-planar. Also the lateral diffusion of the dyes is too large for very small pixels, being at least the thickness of the mordant layer, typically several microns.
Brault, U.S. Pat. No. 4,081,277 teaches repeated thermal dye transfer into a receiving layer using a photoresist mask, but this method also suffers from poor lateral definition of the dye and has not found use in small pixels. Drexhage, U.S. Pat. No. 4,247,799, discloses a single dyed polymer layer that is photobleachable, so that in principal regions of different colors can be formed by optical exposure at different wavelengths, but the edges of the color filter elements are not abruptly defined by this process due to light scatter and beam focus at the scale of a few tenths of a micron. This method has not found acceptance due to the need for special exposure equipment and to the difficulty of finding dyes which are both photobleachable and whose spectral properties are optimal for imagers. Pace and Blood, U.S. Pat. No. 4,764,670, discuss a two layer subtractive color system which provides precise control of density and hue and reduce the number of colors needed in each layer. Although their schematic illustrations indicate perfect registration of the color filter elements, no method is provided for achieving perfect registration, there being still a need for photolithographic alignment to define the lateral extent of the first and the second dyed layer and their overlap, nor is mordant swelling prevented. While this technique is advantageous for large pixel sizes, lack of planarity is disadvantageous for small image sensors, for example for pixels less than 10 microns in size. Snow et al, U.S. Pat. No. 4,876,167, describes a variety of photo-crosslinkable mordants to enable deposition of dyes in specific regions of a mordant which have been exposed optically, but these materials also suffer from lack of adequate spatial resolution, residual dye instability, and from swelling of the mordant. Blazey, U.S. Pat. No. 4,307,165, and Whitmore, U.S. Pat. No. 4,387,146 disclose the confinement of dyes in cells, but the means of fabrication using organic cell structures with thick cell walls formed photolithographically or by embossing has not proven advantageous for small pixel imagers due to the gap between dyed regions, and to the fact that thin walls are prone to distortion when they are not supported by material between them, as is the case for the cells described U.S. Pat. No. 4,387,146 and U.S. Pat. No. 4,307,165. This is particularly true for semiconductor process environments due to the use of fluid baths whose surface tension can distort unsupported thin walls, as is well known in the art.